This invention relates to a magnetic detector which may be employed as a magnetic rotary encoder or linear scale.
A conventional magnetic detector, as shown in FIG. 11, comprises: a magnetic drum 1; and a magnetic resistance element 2; and a signal processing circuit 3. The magnetic drum 1 is a magnetic recording medium which is magnetized in such a manner that N and S poles occur alternately at certain intervals; that is, it is magnetized with a repetitive signal having a certain wavelength .lambda.. The magnetic resistance element 2, as shown in FIG. 12, is made up of a plurality of magnetic resistance strips 4, 5, 6 and 7 (hereinafter referred to as "MR strips", when applicable). The MR strips 4, 5, 6 and 7 face the magnetic drum, and are arranged at intervals of .lambda./4, .lambda./8 and .lambda./4 in the direction of rotation of the magnetic drum 1. In the case of a current drive system in which the MR strips 4 through 7 are driven with current, the signal processing circuit 3 comprises: constant current sources 8, 9, 10 and 11; and differential amplifiers 12 and 13.
First ends of the MR strips 4 through 7 are connected to a DC source having a voltage Vc, and the other ends are connected to the constant current sources 8 through 11, respectively. In response to magnetic signals from the magnetic drum 1, the resistances R.sub.11 (x), R.sub.12 (x), R.sub.13 (x) and R.sub.14 (x) of the MR strips 4 through 7 change with the relative movement distance of the magnetic drum 1 and the magnetic resistance element 2. The differential amplifier 12 performs the differential amplification of the voltages V.sub.11 (x) and V.sub.12 (x) of the constant current sources 8 and 9, and the differential amplifier 13 performs the differential amplification of the voltages V.sub.13 (x) and V.sub.14 (x) of the constant current source 10 and 11. As the magnetic drum 1 and the magnetic resistance element 2 are moved relative to each other in the direction of the arrow, the resistances R.sub.11 (x) and R.sub.12 (x) of the MR strips 4 and 5 and the resistances R.sub.13 (x) and R.sub.14 (x) of the MR strips 6 and 7 change in opposite phase to each other, and the resistances R.sub.11 (x) and R.sub.13 (x) of the MR strips 4 and 6 change with a 90.degree. phase difference, as a result of which the differential amplifiers 12 and 13 output sine waves e.sub.11 (x) and e.sub.12 (x) which are different by 90.degree. in phase from each other.
In the above-described magnetic detector, the magnetic resistance element 2 is formed with the MR strips 4 through 7 as unitary segments. Therefore, when an external disturbance magnetic field is applied to the magnetic detector, then the output signals of the differential amplifiers 12 and 13 will include noise components. This will be described in more detail. Examples of the external disturbance magnetic field are magnetic noises directly induced by an electric motor, or caused by the electro-magnetic braking of an electric motor. The noises are low frequency noises whose periods are 100 to 1000 times as high as that of the output magnetic signal of the magnetic recording medium. Therefore, when such an external disturbance magnetic field is applied to the magnetic detector, a bias magnetic field corresponding to the external disturbance magnetic flux density Bo of the external disturbance magnetic field, as shown in part (b) of FIG. 13, is applied to the MR strips.
When the output currents of the constant current sources 8 through 11 are represented by I, then the following equations (1) through (4) are established: EQU V.sub.11 (x)=V.sub.c -R.sub.11 (x)I (1) EQU V.sub.12 (x)=V.sub.c -R.sub.12 (x)I (2) EQU e.sub.11 (x)=V.sub.11 (x)-V.sub.12 (x) (3) EQU R.sub.12 (x)=R.sub.11 (x+.lambda.4) (4)
where the amplification factors of the differential amplifiers 12 and 13 are set to one (1).
The following magnetic flux density B.sub.11 (x) is applied to the MR strip 4: EQU B.sub.11 (x)=Bo+Ba sin (2.pi./.lambda.)x (5)
where Bo is the external disturbance magnetic flux density, and Ba is the amplitude of the signal magnetic flux density produced by the magnetic record signal of the magnetic drum.
The resistance change rate of the MR strip 4 with respect to the applied magnetic flux density, approximating a quadratic curve as shown in part (a) of FIG. 13, is assumed to be as follows: EQU .rho.(B.sub.11 (x))=A{B.sub.11 (x)}.sup.2 ( 6)
where .rho. is the rate of decrease.
When the magnetic flux density B.sub.11 (x) is applied to the MR strip 4 as shown in part (b) of FIG. 13, the resistance change rate .rho.(B.sub.11 (x)) of the MR strip 4 changes as shown in part (c) of FIG. 13. The relation of the resistance R.sub.11 (x) of the MR strip 4 and the applied magnetic flux density is as follows: EQU R.sub.11 (x)=Ro{1-.rho.(B.sub.11 (x))} (7)
where Ro is the resistance of the MR strip 4 when the magnetic flux density applied thereto is 0 (gauss). As is apparent from equation (7), when the magnetic flux density B.sub.11 (x) is applied to the MR strip 4, the resistance R.sub.11 (x) of the strip 4 is decreased from R.sub.0 as much as R.sub.OR.sup..rho..sub.(B11 (x)).
When equations (5) and (6) are inserted into equation (7), then ##EQU1##
From equations (8) and (4), ##EQU2##
When equations (1), (2), (8) and (9) are inserted in equation (3), the output signal e.sub.11 (x) of the differential amplifier 12 is as follows: ##EQU3##
In equation (10), the first term of the right side indicates the signal component which repeats every x=.lambda./2, and the second term of the right side indicates the noise component whose frequency is a half (1/2) of the frequency of the signal which repeats every x=.lambda.. Therefore, the amplitude of the noise component is 2 .sqroot.2RoIAB.sub.0 Ba, and the noise component is zero (0) when the external disturbance magnetic flux density B.sub.0 is zero (0). Equation (10) means that, with B.sub.0 .noteq.0, the noise component whose frequency is a half of that of the signal which repeats every x=.lambda. is superposed on the signal component. The ratio of amplitude of the signal e.sub.11 (x) and the noise component is: ##EQU4## As is apparent from equation (11), the magnetic flux density ratio B.sub.0 /Ba is converted into a voltage signal, and the S/N ratio is amplified by a factor of 2.sqroot.2.
As is apparent from the above description, the conventional magnetic detector is disadvantageous in that, when an external disturbance magnetic field is applied thereto, the output signals of the differential amplifiers 12 and 13 will include the noise components, and therefore in processing them into square waves, the duty ratio changes every period.